A two-component developing method has been used widely as an excellent developing method in an electrophotographic developing method, but it still has some problems even today. Among the problems, there is a strong demand for a two-component developer which can be used for a long time and their properties are constant and stabilized for their total service period.
Also proposed has been a photoreceptor possessing a surface protective layer in which particles are dispersed for longer lifetime of the photoreceptor as a promising approach to obtain high image quality and longer lifetime in the entire conventional image forming processes. Further, concerning measures of high image quality and failure in lack of line image, the following proposals are given since contributions from the photoreceptor surface are high.
(1) A lubricant is coated on the photoreceptor surface in order to enrich lubrication of a photoreceptor.
(2) A lubrication providing material is added into a photoreceptor surface protective layer solution to conduct coating.
(3) A lubricant released from toner is brought into the photoreceptor surface in which particles are dispersed.
However, regarding (1), it reflects unfavorably upon measures of downsizing of apparatuses as well as energy saving.
Regarding (2), it is difficult to distribute a material exhibiting a lubrication effect evenly on a surface protective layer with a conventional immersion coating method, and the outermost surface portion of the surface protective layer becomes low in concentration. Therefore, the photoreceptor surface needs to be mechanically polished by the aftertreatment process or the like to expose the material exhibiting a lubrication effect on the photoreceptor surface, whereby high cost can not be avoided (refer to Patent Document 1, for example).
For this reason, it is proposed that particles made of a lubrication providing material (fluorine resin particles, for example) are dispersed on the photoreceptor surface by a CSH coating method (a slide hopper coating method) to disperse particles evenly on the outermost surface of the photoreceptor, but scratches are easily generated, whereby failure in streak-shaped noise is generated, though extension effects from the lubrication providing material are highly produced.
Regarding (3), there appear problems such as lowering of electrification, generation of powder smoke and so forth when an addition amount of a lubricant into toner is too much since the addition amount is limited, though a certain level of a releasing property is effective. Further, a supplying amount of a lubricant into the photoreceptor surface is changed when a print ratio during image formation is varied, whereby the releasing property of the photoreceptor surface is varied, resulting in unfavorably influenced image quality.
On the other hand, in recent years, studies on small sized toner particles have been actively done for the purpose of obtaining high image quality. In this case, there were problems such as lowered toner fluidity, lack of stability in frictional electrification with carrier and so forth. In order to solve these problems, the amount of external additives added for obtaining the toner fluidity tends to be increased. However, in the case of a large amount of external additives, the amount transferring into carrier during storage as well as developing treatment is also increased, whereby stable images tend to be difficult to be obtained for a long duration since charge providing performance of carrier is lowered for the foregoing reason, and there appears a large difference of developer properties between at the initial stage of developer preparation and after aging of the developer.
Consequently, proposed is a developing system (so-called trickle developing system) in which when toner is newly supplied in response to toner consumed via development, carrier is added, and carrier in a developing device is interchanged little by little by gradually removing a usually excessive amount of developer to stabilize developer concentration via suppression of change in electrification (refer to Patent Document 2, for example).
In the foregoing trickle development, the fluctuation of the amount of charge has been tried to be suppressed by gradually replacing the carrier with a fresh carrier. However, in combination with the toner having a small diameter, a phenomenon in such a way that the amount of charge sometimes becomes smaller is often generated. This reason might be considered as follow; in the case of the toner having a small diameter wherein the surface area per a unit volume is large, the amount of external additives to be added is large, and thereby, the amount of the external additives transferred to the newly supplied carrier is also large, resulting in decrease of a charge providing property of carrier.
Specifically, when printing of many sheets consuming a large amount of toner is carried out, the amounts of toner and carrier to be supplied become greater, and the charge providing property caused by transfer of external additives to the newly supplied carrier is easy to be generated, whereby undesired image fog or unevenness in image density were often generated.    (Patent Document 1) Japanese Patent O.P.I. Publication No. 5265243    (Patent Document 2) Japanese Patent Examined Publication No. 2-21591